An example of a conventional positioning control method will be described with reference to FIG. 6. FIG. 6 shows an example of a configuration for positioning control in the direction of only one axis. In FIG. 6, 21 represents a motor; 22 represents a motor position detector; 23 represents a workpiece; 24 represents a table; 25 represents a base (surface plate); 26 represents a motor controller; 27 represents a motor driving signal; 28 represents a motor position detection signal; 29 represents a table position detection signal; 30 represents a reduction gear; 31 represents a ball screw; 32 represents a nut supporting an end of the ball screw; 33 represents an anti-vibration pad; and 34 represents a position target value signal.
In FIG. 6, the position target value signal 34 is supplied to the motor controller 26, and the motor controller 26 controls the position of the table 24 having the workpiece 23 placed thereon such that it will reach the position target value using the motor position detection signal 28 and table position detection signal 29.
FIG. 7 shows a block diagram of an example of a configuration for control in the motor controller 26. In FIG. 7, 36 represents a table position compensator, and 35 represents a motor control compensator. The table position compensator 36 evaluates the table position detection signal 29 and the table position target value to determine a value to be output to the motor position compensator 35. The motor position compensator 35 evaluates the output of the motor position detection signal 28 and the output of the table position compensator 36 and outputs a drive signal to the motor 21. Many conventional motor controllers perform table positioning by positioning a motor using only the motor position detection signal 28. While the above-described example has referred to a ball screw as an example of a table driving mechanism, an increasing number of apparatuses utilize a linear motor as driving means recently. In general, a motor controller uses also the table position detection signal in such cases. As thus described, the position of a table has been made equal to a target value to put a workpiece secured to the table in a desired target position.
However, a recent trend toward reduction of table moving time in order to improve production yield has resulted in higher table moving speeds and greater propelling forces for driving a table. As a result, a greater reaction force is applied to the base from the table when the propelling force occurs, which has resulted in vibration of the base. This phenomenon is significant especially when a linear motor is used as means for driving the table. Since the displacement and phase of vibration is different between the table and the base during base vibration, the base vibration remains even after a table driving operation to cause fluctuation of the table position, which has resulted in a problem in that the time between the beginning and end of table driving can not be reduced.
There is another problem in that methods for suppressing base vibration through improvement of existing facility have small feasibility because the size and cost of the facility will be increased when base vibration is to be measured.